Extreme Ultraviolet (EUV) Mask, Method Of Fabricating The EUV Mask And Method Of Inspecting The EUV Mask

ABSTRACT

An out-of-band (OoB) suppression layer is applied on a reflective multiplayer (ML) coating, so as to avoid the OoB reflection and to enhance the optical contrast at 13.5 nm A material having a low reflectivity at wavelength of 193-257 nm, for example, silicon carbide (SiC), is used as the OoB suppression layer. A method of fabricating an EUV mask having the OoB suppression layer and a method of inspecting an EUV mask having the OoB suppression are also provided.

BACKGROUND

In the manufacture of integrated circuits (IC), patterns representingdifferent layers of the IC are fabricated using a series of reusablephotomasks (“masks”) to transfer the design of each layer of the IC ontoa semiconductor substrate during the manufacturing process in aphotolithography process. These layers are built up using a sequence ofprocesses and resulted in transistors and electrical circuits. However,as the IC sizes continue to shrink, meeting accuracy requirements aswell as reliability in multiple layer fabrication has becomeincreasingly more difficult.

Photolithography uses an imaging system that directs radiation onto thephotomask and then projects a shrunken image of the photomask onto asemiconductor wafer covered with photoresist. The radiation used in thephotolithography may be at any suitable wavelength, with the resolutionof the system increasing with decreasing wavelength. Deep ultraviolet(DUV) light with a radiation at a wavelength of 248 or 193 nanometers(nm) has been widely used for exposure through a transmissive mask.However, with the shrinkage in IC size, extreme ultraviolet (EUV)lithography with a typical wavelength of 13.5 nm becomes one of theleading technologies for 16 nm and smaller node device patterning.

An EUV mask utilized for the EUV lithography is a layered structureincluding a Bragg mirror deposited on a substrate. On the substrate, areflective multilayer stack, which is formed by sequentially stackingmaterials having different optical properties, is used to achieve a highEUV light reflectance. The pattern is formed from absorptive features orlines etched into the EUV mask. The reflective multiplayer stack is atype of Bragg reflector that reflects light at a selected wavelengththrough constructive interference. The thicknesses of the alternatinglayers are tuned to maximize the constructive interference (Braggreflection) of the EUV light reflected at each interface and to minimizethe overall absorption of the EUV light. The multiplayer coating canachieve about 60 to 75% reflectivity at the peak radiation wavelength.The EUV Lithography process may lack spectral purity for its lightsources, meaning the light sources may produce undesirable out-of-band(OoB) radiation, i.e., radiation of an undesirable bandwidth, forexample, between 193 nanometers (nm) to 257 nm. Existing photoresistmaterials may be sensitive to the OoB radiation and may absorb suchradiation. This would result in reduced contrast and hence degradationof imaging performance.

On the other hand, the EUV masks require frequent cleaning to reduce oreliminate defects during the optical lithography operation. The cleaningis typically performed at an elevated temperature to enable and/orenhance the efficiency of the cleaning chemistry. In addition, duringuse the masks are inadvertently heated through exposure with extremeultraviolet light. In this regard, the mask is frequently exposed totemperatures above ambient during the masks lifecycle and is used attemperatures exceeding ambient during normal operation. Consequently,these conditions can cause several types of chemical diffusion andchemical reactions within the multilayer stack of the Bragg mirror.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure are best understood from thefollowing detailed description when read with the accompanying figures.It is emphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a block diagram of a photolithography imaging system that usesa mask in processing a wafer.

FIG. 2 is a cross-sectional view schematically illustrating an EUV maskaccording to various embodiments of the present disclosure.

FIG. 3A is a cross-sectional view schematically illustrating maskaccording to various embodiments of the present disclosure.

FIG. 3B is a cross-sectional view schematically illustrating an EUV maskaccording to various embodiments of the present disclosure.

FIG. 3C is a cross-sectional view schematically illustrating an EUV maskaccording to various embodiments of the present disclosure.

FIG. 3D are diagrammatic cross-sectional side views of the EUV mask ofFIG. 2A at various stages of fabrication according to variousembodiments of the present disclosure.

FIG. 4 is a flowchart illustrating a method of fabricating an EUV maskaccording to various embodiments of the present disclosure.

FIG. 5 is a flowchart illustrating a method of inspecting an EUV maskaccording to various embodiments of the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the disclosure. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the formation of a first feature over or on a second feature in thedescription that follows may include embodiments in which the first andsecond features are formed in direct contact, and may also includeembodiments in which additional features may be formed interposing thefirst and second features, such that the first and second features maynot be in direct contact. Various features may be arbitrarily drawn indifferent scales for the sake of simplicity and clarity.

The singular forms “a,” “an” and “the” used herein include pluralreferents unless the context clearly dictates otherwise. Therefore,reference to, for example, a gate stack includes embodiments having twoor more such gate stacks, unless the context clearly indicatesotherwise. Reference throughout this specification to “one embodiment”or “an embodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present disclosure. Therefore, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Further, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments. It should be appreciated that the followingfigures are not drawn to scale; rather, these figures are intended forillustration.

The EUV lithography is an exposure technique using EUV light. The EUVlight refers a ray having a wavelength in a soft X-ray region or avacuum ultraviolet ray region. Specifically, the EUV light has awavelength of about 10 to 20 nm, particularly about 13.5 nm±0.3 nm. Inthe EUV lithography, illumination is cast on the EUV mask at an angle,e.g., 5° relative to the axis perpendicular to a plane of the mask.Challenges exist in an optics system of the EUV lithography fortransferring a pattern to the wafer, including the optics deformation,contamination, source stability, dose uniformity, shot noise, flare(stray lights), optical contrast, etc.

As to the optical contrast of the pattern formed from the EUV mask at13.5 nm, the reflective multiplayer stack also reflects radiation ofbandwidths that are out of a desired bandwidth of the EUV band. Forexample, radiation having a bandwidth in a range between about 193-247nm is considered undesirable Out-of-Band (OoB) radiation for EUVlithography processes. The OoB radiation is included in light generatedfrom a light source of EUV exposure apparatus. As a result, theabsorption of such OoB radiation results in reduced optical contrast anddegradation of imaging performance of conventional photoresistmaterials. Exposure of the EUV photoresist to OoB radiation typicallyresults in unwanted background exposure of the resist called “flare.”Flare among other things hurts the resolution of the resist, reducingcontrast with respect to unexposed areas, and compromising the abilityto etch patterns of sufficiently small sizes.

According to various embodiments of the present disclosure, an OoBsuppression layer is applied on a reflective multiplayer (ML) coating,so as to avoid the OoB reflection and thus enhance the optical contrastat 13.5 nm. A material having a low reflectivity at wavelength of193-257 nm, for example silicon carbide (SiC), is used as the OoBsuppression layer. In some embodiments, the OoB suppression layer is acomposite layer having two layers, a SiC layer and a Mo layer. Accordingto various embodiments of the present disclosure, a buffer layer made ofSiC can be deposited over the reflective ML coating or the OoBsuppression layer to enhance the optical contrast at 13.5 nm.

According to various embodiments of the present disclosure, a method ofinspecting an EUV mask is provided. In the EUV lithography process, theEUV masks require frequent cleaning to reduce or eliminate defectsduring the optical lithography operation. In the cleaning process, themask is frequently exposed to temperatures above ambient during themasks lifecycle and is used at temperatures exceeding ambient duringnormal operation. Consequently, defects occur within the reflective MLcoating and degrade the performance of the EUV mask. In variousembodiments of the present disclosure, the EUV mask including the OoBsuppression layer also has an improved optical contrast at a radiationof wavelength at 193 nm. Because of the enhanced optical contrast atwavelength at 193 nm, the method according to the various embodiments ofthe present disclosure provides better detecting efficacy of the defectsor particles on the EUV mask during the cleaning process or usageoperation. Further, the OoB suppression layer of SiC has a betterresistance than the conventional materials (e.g., Si) to attacks of thechemicals used in the EUV lithography process or the cleaning process.

Photolithography uses an imaging system that directs radiation onto amask having a pattern and then projects a reduced image of that maskonto a semiconductor wafer covered with photoresist. The radiation usedin photolithography may be at any suitable wavelength, with theresolution of the system increasing with decreasing wavelength. Theability to print smaller features onto the semiconductor wafer improvesas the resolution increases. Concerning EUV lithography, it is based onexposure with the portion of the electromagnetic spectrum having awavelength of 10-15 nanometers. An EUV step-and-scan tool may have a4-mirror, 4×-reduction projection system with a 0.10 Numerical Aperture(NA). Exposure is accomplished by stepping fields over a wafer andscanning the EUV mask across each field. Various types of masks used inphotolithography include such as binary mask, alternating phase-shiftmask, and attenuated phase-shift mask (att-PSM), as well as varioushybrid mask types. The EUV mask may be fabricated by exposing anddeveloping the photoresist layer of the blank substrate to form aphotoresist pattern and by etching the absorption layer and the bufferlayer using the photoresist pattern as an etch mask to form anabsorption layer pattern. If the absorption layer pattern is formed tohave a critical dimension (CD), for example, of a size that is differentfrom the design CD value, it may be difficult to compensate the CD ofthe absorption layer pattern. A CD of 50-70 MD may be achieved with adepth of focus (DOF) of about 1 micrometer (um). Alternatively, a6-mirror, 4X-reduction projection system may be applied with a 0.25 NAto print a smaller CD of 20-30 nm, at the expense of a reduced DOF.Other tool designs with a 5X-or a 6X-reduction projection system mayalso be used for EUV lithography.

Referring to FIG. 1, an EUV lithography imaging system 100 includes aradiation source 110, a condenser optics section 120, a projectionoptics section 130, a mask stage 140, and a wafer stage 150. Theradiation source 110 may be any source able to produce radiation in theEUV wavelength range. One example of a suitable radiation source 110 iscreates a plasma when a laser illuminates a gas, such as a supersonicjet of xenon gas. As another example, a suitable radiation source 110may be use bending magnets and undulators associated with synchrotrons.As a further example, a suitable radiation source 110 may be usedischarge sources, which have the potential to provide adequate power inthe desired wavelength range. EUV radiation is strongly absorbed invirtually all transmissive materials, including gases and glasses. Tominimize unwanted absorption, EUV imaging is carried out in near vacuum.

The condenser optics section 120 brings the radiation from the source110 to the mask stage 140. In the EUV lithography imaging system 100,the condenser optics are reflective because EUV radiation is stronglyabsorbed in traditionally transmissive materials such as lenses, whichmay be used in traditional photolithography imaging systems.Accordingly, the condenser optics section 120 includes condenserreflectors or mirrors 125 that collect and focus the radiation from thesource 110 onto the mask stage 140. Any number of condenser mirrors 125may be used, such as, for example, the four shown in FIG. 1.

The mask stage 140 includes a transport stage 146 that scans a mask 142.In the EUV lithography imaging system 100, the mask 142 is reflectivebecause EUV radiation is strongly absorbed in most materials such astransmissive photomasks that are used in traditional photolithographyimaging systems.

The projection optics section 130 reduces the image from the mask 140 inthe mask stage 140 and forms the image onto wafer 152 in the wafer stage150. In the EUV lithography imaging system 100, the projection opticsare reflective because of the absorption associated with EUV radiation.Accordingly, the projection optics section 130 includes reflectors ormirrors 135 that project radiation reflected from the mask 140 onto thewafer. The reflectance spectrum of the mask 142 may be matched to thatof the mirrors in the projection optics section 130. The term“projection optics” used herein should be broadly interpreted asencompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used.

The wafer stage 150 includes a transport stage 156 that scans asemiconductor wafer 152 in synchrony with the mask 142 and steps thewafer 152 into a position to accept a next image from the mask 142.During operation, a semiconductor wafer 152 mounted to the transportstage 156. The projection optics convey the radiation light with apattern in its cross-section to create a pattern in a target portion ofthe wafer 152. It should be noted that the pattern conveyed to theradiation light may not exactly correspond to the desired pattern in thetarget portion of the wafer, for example if the pattern includesphase-shifting features or shadows. Generally, the pattern conveyed tothe radiation light will correspond to a particular functional layer ina device being created in a target portion of the wafer 152, such as anIC.

FIG. 2 is a schematic cross-sectional view of an EUV mask 200, having apatterned absorber layer 240 according to various embodiments of thepresent disclosure. The EUV mask 200 includes a substrate 210, areflective multilayer (ML) coating 220 for reflecting EUV light, anout-of-band (OoB) layer consisting of a first layer 226 and a secondlayer 224, and a buffer layer 230. In various embodiments, a cappinglayer 228 made of SiC may be deposited on the OoB suppression layer224/226 prior to the forming of the buffer layer 230. In someembodiments, the buffer layer 230 acts as a capping layer, such that thecapping layer of 228 is the same layer of the buffer layer 230.According to embodiments of the present disclosure, the patternedabsorber layer 240 is a dual layer of consisting of a first layer 242and a second layer 244.

During the EUV lithography process, up to about 40% of the EUV light isabsorbed by the EUV mask. Thermal expansion caused by the heating leadsto a large image distortion that may exceed the error tolerance. Lowthermal expansion material (LTEM) has been used as the substratematerial for the substrate of the EUV masks. The substrate 210 may havea low thermal expansion coefficient (for example, the thermal expansioncoefficient within a temperature range of from 19° C. to 27° C. is0±1.0×10⁻⁷/° C. In various embodiments, the thermal expansioncoefficient is 0±0.3×10⁻⁷/° C., 0±0.2×10⁻⁷/° C., 0±0.1×10⁻⁷/° C., or0±0.05×10⁻⁷/° C. A glass having a low thermal expansion coefficient,such as a β quartz may be used as the substrate 210. Further, a filmsuch as a stress correcting film (not shown) may be formed on thesubstrate 210.

The reflective ML coating 220 of the EUV mask is used to achieve a highEUV light reflectance. The reflective ML coating 220 is a type of Braggreflector that reflects light at a selected wavelength throughconstructive interference. The selection of materials in the ML coating220 depends on the radiation wavelength (λ) to be reflected. Each layerof the ML coating 220 has a thickness of about one quarter of λ. Inparticular, the thickness of the respective layers of the ML coating 220depends on the radiation wavelength and the incidence angle of theradiation light. For EUV, the λ is 13.5 nm and the incidence angle isabout 5 degrees. Using many reflecting film pairs, over 60% reflectancefor light having a wavelength in the vicinity of 13.5 nm is achieved.The thicknesses of the alternating layers are tuned to maximize theconstructive interference (Bragg reflection) of the EUV light reflectedat each interface and to minimize the overall absorption of the EUVlight. The ML coating 220 can achieve about 60-75% reflectivity at thepeak radiation wavelength. The reflective ML coating 220 is formed bysequentially stacking materials 222/224 having different opticalproperties. The Bragg reflection occurs at the interface of thematerials 222/224. The reflectivity of the reflective ML coating 220 isproportional to the square of the difference between the refractiveindexes (real parts of complex refractive indexes) of the two materials222/224 that are alternately stacked. In addition, the wavelength andmaximum reflectivity of the reflected EUV light are determined basedupon the kinds of the materials in 222/224. In various embodiments, thereflective ML coating 220 has 30 pairs to about 60 pairs of alternatinglayers of a low index of refraction material 224 and a high index ofrefraction material 222. For example, 40 pairs of the alternative layers222/224 of the first ML film 320 are deposited in which the high indexof refraction material 222 may be formed from about 2.8 nm thickMolybdenum (Mo) while the low index of refraction material 224 may beformed from about 4.1 nm thick Silicon (Si).

According to various embodiments of the present disclosure, the OoBlayer consisting of a first layer 226 and a second layer 224 is formedon a top surface of the reflective ML coating 220. The first layer 226is made of a material having a low reflectivity of the radiation atwavelength of 193-257 nm, and a low refraction index than that of thematerial for the second layer 224. The term “low'” reflectivity refersto the material having a value of reflectivity to the radiation at193-257 nm lower than that of materials used in the reflective MLcoating 220. In embodiments, the first layer 226 may be made of SiC andthe second layer 224 be made of Mo. Accordingly, the EUV mask of FIG. 2reflects less OoB light and thus improves optical contrast in the EUVregion.

A buffer layer 230, such as about 11 nm thick Ruthenium (Ru), may beformed over the top surface of the OoB suppression layer 224/226. insome embodiments, a capping layer 228 made of SiC may be deposited onthe OoB suppression layer 224/226 prior to forming the buffer layer 230.The Mo layer in either the reflective ML coating 220 or the OoBsuppression layer can become oxidized under ambient conditions. Thecapping layer 228 of SiC prevents oxidation of the Mo layer. In certainembodiments, the capping layer 228 of SiC may be the buffer layer 230;that is, the buffer layer 230 is the same layer of the capping layer 228of SiC and acts as the capping layer. In specific embodiments, thecapping layer 228 of SiC has a thickness of 2-7 nm, and for example, athickness of 3 nm.

According to various embodiments of the present disclosure, a patternedabsorber layer 240 is formed over the reflective ML coating 220. Invarious embodiments, the absorber layer 240 has a thickness d in a rangeof 30-70 nm. In some embodiments, the absorber layer 240 is a dual-layerstack having a first layer 242 and a second layer 244. In variousembodiments, more than one dual-layer stack may be deposited over thereflective ML coating 220.

FIGS. 3A-3D are the cross-sectional side views of the EUV mask 200 ofFIG. 2A at various stages of manufacture according to variousembodiments of the present disclosure. For reasons of simplicity, FIGS.3A-3D may only illustrate a part of the EUV mask.

hr FIG. 3A, a substrate 310 with a low defect level and a smooth surfaceis used as the starting material for the EUV mask 200 in the presentdisclosure. The substrate 310 has a low coefficient of thermal expansion(CTE). In some embodiments, the substrate 310 is a glass orglass-ceramic material. For example, the substrate 310 may be formed ofβ-quartz,

Referring to FIG. 3B, a reflective ML coating 320 is formed over thesubstrate 310. The reflective ML coating 320 has about 30-60 pairs ofalternating layers of a low index of refraction material 324 and a highindex of refraction material 322. In some embodiments, the reflective MLcoating 320 has 40 pairs of the alternative layers 322/324. A high indexof refraction material 322 includes elements with high atomic numberwhich tend to scatter EUV light. A low index of refraction material 324includes elements with low atomic number which tend to transmit EUVlight. The reflective ML coating 320 is formed over the substrate 310 byusing ion beam deposition (IBD) or DC magnetron sputtering. Thethickness uniformity should be better than 0.8% across the substrate310. IBD results in less perturbation and fewer defects in the uppersurface of the reflective ML coating 320 because the depositionconditions can usually be optimized to smooth over any defect on thesubstrate 310. In various embodiments, 40 pairs of the alternativelayers 322/324 of the ML coating 320 are deposited in which the highindex of refraction material 322 may be formed from about 3 nm thick Mowhile the low index of refraction material 324 may be formed from about4 nm thick Si. For example, the high index of refraction material 322may be formed from about 2.8 nm thick Mo while the low index ofrefraction material 324 may be formed from about 4.1 nm thick Si.

In various embodiments, in fabricating an EUV mask, a substrate 310 maybe provided having the reflective ML coating 320 thereon. In this caseof the substrate 310 already having the reflective ML coating 320, theoperation in FIG. 3B may be omitted in the method of fabricating the EUVmask according to the embodiments of the present disclosure.

As shown in FIG. 3C, an OoB suppression layer consisting of a firstlayer 326 and a second layer 324 is deposited on the reflective ML layer320. The first layer 326 is made of a material having a low reflectivityof the radiation at wavelength of 193-257 nm, and a low refraction indexthan that of the material for the second layer 324. In embodiments, thefirst layer 326 may be made of SiC and the second layer 324 be made ofMo. In various embodiments, the first layer 326 and the second layer 324may be formed by using ion beam deposition (IBD) or DC magnetronsputtering.

In some embodiments, a capping layer 328 made of SiC may be deposited onthe OoB suppression layer 324/326. Because the Mo layer in either thereflective ML coating 320 or the OoB suppression. layer can becomeoxidized under ambient conditions, the capping, layer 328 of SiCprevents oxidation of the Mo layer. In certain embodiments, the cappinglayer 328 of SiC has a thickness of 2-7 nm, and for example, a thicknessof 3 nm. In some embodiments, the capping layer 328 is made of Ruthenium(Ru). In various embodiments, the first layer 326 and the second layer324 may be formed by using ion beam deposition (IBD) or DC magnetronsputtering

The buffer layer 330 is formed over the OoB suppression layer 324/326 orthe capping layer 328. The buffer layer 330 may have a thickness ofabout 20-60 nm. The buffer layer may be formed from silicon dioxide(SiO₂) or a silicon (Si) layer, In various embodiments, the buffer layermay a Ru capping layer formed at the top of the ML coating 320 toprevent oxidation of Mo by exposure to the environment. The buffer layer330 may be low temperature oxide (LTO) as SiO₂, or other materials, suchas silicon oxynitride (SiOxNy) or carbon (C). The buffer layer 330 mayact later as an etch stop layer for patterning of the overlying absorber340 formed in the following operation. In some embodiments, the cappinglayer 328 of SiC may be the buffer layer 330; that is, the buffer layer330 is the same layer of the capping layer 328 of SiC and acts as thecapping layer. Furthermore, the buffer layer 330 may also serve later asa sacrificial layer for focused ion beam (FIB) repair of defects in theabsorber 340. The buffer layer 330 may be deposited by a suitableprocess such as magnetron sputtering and ion beam sputtering.

Referring to FIG. 3D, an absorber layer 340 is formed over the bufferlayer 330 or the capping layer 328 according to various embodiments ofthe present disclosure. In embodiments, the absorber layer 340 has atotal thickness d raging from 30-70 nm. The absorber layer 340 may bedeposited by RF sputtering, DC sputtering, ion beam deposition (IBD) oratomic layer chemical vapor deposition (ALD). In various embodiments,shown in FIG. 3D, the absorber layer 340 is a dual-layer stack includinga first layer 342 and a second layer 344 [not shown in FIG. 3D] made ofhighly absorptive materials to the radiation of wavelength at 13.5 nm.Patterning the absorber layer 340 includes forming a photoresist patternover the absorber layer 340 in an absorption region, etching theabsorber layer 340 by using the photoresist pattern as an etch mask toform an absorber pattern, and removing the photoresist pattern. Inparticular, the absorber layer 340 may be covered with aradiation-sensitive layer, such as photoresist, that is coated, exposed,and developed with a desired pattern. The photoresist pattern has athickness of about 160-640 nm. As desired, a chemically-amplified resist(CAR) may be used. Depending on the photoresist pattern used, exposureis performed on an electron beam (e-beam) writer or a laser writer.Reactive ion etch may be used. For example, an absorber layer 340 may bedry etched with a gas that contains chlorine, such as Cl₂ or BCl₃, orwith a gas that contains fluorine, such as NF₃. Argon (Ar) may be usedas a carrier gas. In some cases, oxygen (O₂) may also be included ascarrier. The etch rate and the etch selectivity depend on the etchantgas, etchant flow rate, power, pressure, and substrate temperature. Thebuffer layer 330 may serve as an etch stop layer to help achieve a goodetch profile in the overlying absorber layer 340. The buffer layer 330protects the underlying reflective ML coating 320 from damage duringetch of the absorber layer 340.

FIG. 4 is a flowchart illustrating a method of fabricating a method offabricating an EUV mask according to various embodiments of the presentdisclosure. The operations are explained in the cross-sectional sideviews of a portion of the EUV mask 200 from FIGS. 3A to 3D at variousfabrication stages according to various embodiments of the presentdisclosure. It is understood that FIGS. 3A-3D have been simplified for abetter understanding of the inventive concepts of the presentdisclosure.

In FIG. 3A, a substrate 310 is provided in operation 402. Referring toFIG. 3A, the substrate 310 is made of a material having a lowcoefficient of thermal expansion (CTE). For example, the substrate 310may be formed of β-quartz.

Referring to the operation 404, a reflective ML coating 320 is depositedover the substrate 310. In FIG. 3B, the reflective ML coating 320 hasabout 30-60 pairs of alternating layers of a low index of refractionmaterial 322 and a high index of refraction material 324. Inembodiments, the reflective ML coating 320 has 40 pairs of thealternative layers 322/324.

As various embodiments, in fabricating an EUV mask, a substrate 310 maybe provided having the reflective ML coating 320 thereon. In this caseof the substrate 310 with the reflective ML coating 320, the operation404 in FIG. 4 may be omitted in the method of fabricating the EUV maskaccording to the embodiments of the present disclosure.

In embodiments, the method of fabricating the EUV mask further includesan operation 406 of depositing an OoB suppression layer on the MLcoating in FIG. 4. Referring to FIG. 3C, the OoB suppression layerconsists of a first layer 326 and a second layer 324. In embodiments,the first layer 326 is made of a material having a low reflectivity ofthe radiation at wavelength of 193-257 nm, and a low refraction indexthan that of the material for the second layer 324. In embodiments, thefirst layer 326 may be made of SiC and the second layer 324 be made ofMo. Accordingly, the OoB layer exhibit a reduced OoB light in view of13.5 nm and an improved optical contrast as far as the radiation at193-257 nm wavelength is concerned.

Still referring to FIG. 3C, in embodiments, a capping layer 328 made ofSiC may be deposited on the OoB suppression layer 324/326. Because theMo layer in either the reflective ML coating 320 or the OoB suppressionlayer can be oxidized under ambient conditions, the capping layer 328 ofSiC prevents oxidation of the Mo layer. In embodiments, the cappinglayer 328 of SiC has a thickness of 2-7 nm, and for example, a thicknessof 3 nm. In various embodiments, a buffer layer 330 is formed over theOoB suppression layer 324/326 or the capping layer 328, The buffer layer330 may have a thickness of about 20-60 nm. The buffer layer may beformed from silicon dioxide (SiO₂) or a silicon (Si) layer. In variousembodiments, the buffer layer may a Ru capping layer formed at the topof the ML coating 320 to prevent oxidation of Mo by exposure to theenvironment. The buffer layer 330 may be low temperature oxide (LTO) asSiO₂, or other materials, such as silicon oxynitride (SiOxNy) or carbon(C). The buffer layer 330 may act later as an etch stop layer forpatterning of the overlying absorber 340 formed in the followingoperation. In embodiments, the capping 3 of SiC may be the buffer layer330; that is, the buffer layer 330 is the same layer of the cappinglayer 328 of SIC and acts as the capping layer.

Referring to the operation 408 of FIG. 4, the absorber layer 340 isformed on the top surface of the OoB suppression layer. In variousembodiments, the absorber 340 may be formed on the buffer layer 330 orthe capping layer 328. The desired pattern on an EUV mask is defined byselectively removing an absorber layer to uncover portions of anunderlying mirror coated on a substrate. According to variousembodiments of the present disclosure, the method of fabricating the EUVmask further includes an operation of forming a resist layer over theabsorber layer, patterning the resist layer to form a trench with atrench width, and etching through the absorber layer 340 to expose thereflective ML coating and removing the resist layer. As a result, thepatterned absorber layer 340 of FIG. 2 is formed accordingly.

FIG. 5 is a flowchart illustrating a method of inspecting an EUV maskaccording to various embodiments of the present disclosure. In operation502, the EUV mask is provided with a substrate, a reflective multilayer(ML) coating over the substrate, an out-of-band (OoB) suppression layermade of, for example, a pair of Mo and SiC layers on the reflective MLcoating, and a capping layer made of SiC on the OoB suppression layer.In operation 504, the EUV mask is radiated with, for example, awavelength of 193 nm, for inspection of a target region. In operation506, foreign matters are detected from diffusely reflected light.Further, the EUV mask is cleaned at suitable process to be reused forthe next EUV lithography process.

According to various embodiments, a contrast of the inspection light atwavelength of 193 nm is in a range of 0.65-0.90. In embodiments, whereinthe EUV mask is an attenuated Phase Shift Mask (att-PSM). [This is afragment. What are you trying to say?] With the method of inspecting theEUV mask according to the embodiments of the present disclosure, the EUVmask including the OoB suppression layer also has an improved opticalcontrast at a radiation of wavelength at 193 nm. Because of the enhancedoptical contrast at wavelength at 193 nm, the EUV mask having an OoBsuppression layer can be inspected with better detecting efficacy of thedefects on or in the EUV mask. Further, the OoB suppression layer of SiChas a better resistance than the conventional materials (e.g., Si) toattacks of the chemicals used in the EUV lithography process or thecleaning process.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. An extreme ultraviolet (EUV) mask, comprising: asubstrate; a reflective multilayer (ML) coating over the substrate; anout-of-band (OoB) suppression layer on the reflective ML coating; and anabsorber layer over the OoB suppression layer, wherein the reflective MLcoating is made of alternating layers of molybdenum (Mo) and silicon(Si), wherein the number of the alternating layers is in a range fromabout 30 pairs to about 60 pairs, and the OOB suppression layer is madeof at least a pair of Mo and silicon carbide (SiC) layers.
 2. The EUVmask of claim 1, wherein the absorber layer is made of a materialselected from the group consisting of TaBN, TaN and CrN.
 3. The EUV maskof claim 1, further comprising a buffer layer between the OoBsuppression layer and the absorber layer, wherein the buffer layer actsas a capping layer.
 4. The EUV mask of claim 3, wherein the buffer layeris made of SiC.
 5. The EUV mask of claim 3, wherein the buffer layer ismade of a material selected from the group consisting of silicon dioxide(SiO₂), silicon oxynitride (SiON), carbon (C), and ruthenium (Ru). 6.The EUV mask of claim 4, wherein the buffer layer of SiC has a thicknessbetween 2 to 5 nm.
 7. The EUV mask of claim 5, wherein the substrate ismade of a low thermal expansion material.
 8. The EUV mask of claim 7,wherein the substrate is made of quartz.
 9. The EUV mask of claim 1,wherein the absorber layer comprises at least one etch opening throughwhich the reflective ML coating is exposed.
 10. A method of fabricatingan extreme ultraviolet (EUV) mask comprising: providing a substrate;depositing a reflective multi-layer (ML) coating over the substrate,wherein the reflective ML coating is made of alternating layers ofmolybdenum (Mo) and silicon (Si), wherein the number of the alternatinglayers is in a range from about 30 pairs to about 60 pairs; depositingan out-of-band (OoB) suppression layer on the reflective ML coating,wherein the OOB suppression layer is made of at least a pair of Mo andsilicon carbide (SiC) layers; and forming an absorber layer over the OoBsuppression layer.
 11. The method of claim 10 further comprising forminga buffer layer over the OoB suppression layer before forming theabsorber layer, and the buffer layer acts as a capping layer.
 12. Themethod of claim 11, wherein forming the buffer layer is forming thebuffer layer of SiC.
 13. The method of claim 12, wherein the bufferlayer of SiC has a thickness between 2 to 5 nm
 14. The method of claim10, further comprising: forming a resist layer over the absorber layer,and patterning the resist layer to form a trench with a trench width;etching through the absorber layer, etching through the buffer layer toexpose the OoB suppression layer; and removing the resist layer.
 15. Themethod of claim 14, wherein etching through the absorber layer and thebuffer layer comprise introducing a chlorine plasma, an oxygen plasma,or both of the chlorine and oxygen plasma.
 16. The method of claim 10,forming the absorber layer is forming the absorber layer with a materialselected from the group consisting of TaBN, TaN and CrN.
 17. The methodof claim 10, depositing the buffer layer is depositing the buffer with amaterial selected from the group consisting of silicon dioxide (SiO₂),silicon oxynitride (SiON), carbon (C), and ruthenium (Ru).
 18. A methodof inspecting an EUV mask, comprising: providing the EUV mask includinga substrate, a reflective multilayer (ML) coating over the substrate, anout-of-band (OoB) suppression layer made of a pair of Mo and SiC layerson the reflective ML coating, and a capping layer made of SiC on the OoBsuppression layer; irradiating the EUV mask to be inspected withinspection light to illuminate a target region; detecting foreignmatters from diffusely reflected light; and cleaning and reusing the EUVmask.
 19. The method of claim 18, wherein a contrast of the inspectionlight at wavelength of 193 nm is in a range of 0.65-0.90.
 20. The methodof claim 18, wherein the EUV mask is an attenuated Phase Shift Mask(att-PSM).